There are several possibilities. Your orange tree may need watering at least once a week during the hottest months of the year. How are you watering your tree? Are you using flood irrigation that extends beyond the canopy? Is it getting enough water?
Second, how are you fertilizing the tree? Is it getting the right nutrients?
Third, are there too many fruit on the branches? Cutting back the number of fruit by one-half when they are small will allow them to get bigger.

There are many references on the Internet about caring for orange trees.